The libssh2_sftp_write function wrapped by this method has a complex and quite difficult (if not impossible at all) to use API.
It tries to hide the packet pipelining being done under the hood in order to attain decent throughput.

Net::SSH2 can not hide that complexity without negatively affecting the transmission speed so it provides just a thin wrapper for that library function.

An excerpt from libssh2_sftp_write manual page follows:

WRITE AHEAD
Starting in libssh2 version 1.2.8, the default behavior of libssh2
is to create several smaller outgoing packets for all data you pass
to this function and it will return a positive number as soon as the
first packet is acknowledged from the server.
This has the effect that sometimes more data has been sent off but
isn't acked yet when this function returns, and when this function
is subsequently called again to write more data, libssh2 will
immediately figure out that the data is already received remotely.
In most normal situation this should not cause any problems, but it
should be noted that if you've once called libssh2_sftp_write() with
data and it returns short, you MUST still assume that the rest of
the data might've been cached so you need to make sure you don't
alter that data and think that the version you have in your next
function invoke will be detected or used.
The reason for this funny behavior is that SFTP can only send 32K
data in each packet and it gets all packets acked individually. This
means we cannot use a simple serial approach if we want to reach
high performance even on high latency connections. And we want that.

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself, either Perl version 5.8.0 or, at your option, any later version of Perl 5 you may have available.

The documentation for this package contains and excerpt from libssh2 manual pages. You can consult the license of the libssh2 project for the conditions regulating the copyright of that part.